Environment Controls Gene Expression: Sex Determination and the Onset of Genetic Disorders

In this day and age, it seems that genes are "everything" when it comes to determining the characteristics of an organism. However, the importance of the environment in biology cannot be denied. For example, nearly everyone knows that a dry season can ruin crop yields. However, digging a little deeper reveals even more intriguing examples of environmental influence on organisms. For instance, research has shown that the sex of some species of reptiles is influenced by the temperature at which the reptiles' eggs are incubated during development. This kind of observation presents an apparent paradox, because sex is usually regarded as being genetically—not environmentally—determined. 

Thus, there appear to be certain situations in which the environment affects not only the growth and health of an organism, but also the use or deployment of the organism's genes. Does this mean that genes aren't, in fact, everything? The observation that genetically identical organisms often vary greatly in phenotype clearly shows that gene-environment interaction is indeed an important regulator of phenotypic variation, including variation related to a number of diseases. In fact, the impact of environmental factors on disease etiology (or causation) has gained greater recognition in recent years. This has led to the conclusion that both genes and environment can affect disease—not just separately, but through direct interaction with each other. This relatively new attitude presents considerable challenges, as well as surprising benefits, for the study of human disease.

The number of combinations of different genotypic variants, environmental conditions, and possible phenotypes is not something researchers can predict. However, the complex interactions of multiple genetic loci with diverse environmental signals suggest that scientists must continue to develop novel methods of studying these situations, such as by simultaneously examining thousands of genes using techniques like microarray technology under different environmental conditions. While we may never be able to predict an exact phenotype, it is clear that when trying to understand biology and human disease, we must consider interactions of genes and environment in our analysis.